1. Field of the Invention
The present invention relates to aqueous polyurethane dispersions and thermosetting compositions containing polyurethane dispersions and dispersions of crosslinked polymeric microparticles. More particularly, the present invention is directed to coating compositions containing aqueous polyurethane dispersions and crosslinked polymeric microparticles used in multi-component composite coating compositions such as primers, pigmented or colored basecoats, and/or transparent topcoats which provide good smoothness and appearance.
2. Background of the Invention
Over the past decade, there has been a concerted effort to reduce atmospheric pollution caused by volatile solvents which are emitted during painting processes. However, it is often difficult to achieve high quality, smooth coating finishes, such as are required in the automotive industry, without the inclusion of organic solvents which contribute greatly to flow and leveling of a coating.
Due to environmental concerns, volatile organic compounds (“VOCs”) and/or hazardous air pollutants (“HAPs”) have come under strict regulation by the government. Therefore, one of the major goals of the coatings industry is to minimize the use of organic solvents by formulating waterborne coating compositions which provide a smooth, high gloss appearance, as well as good physical properties including resistance to acid rain. Unfortunately, many waterborne coating compositions, particularly those containing metallic flake pigments, do not provide acceptable appearance properties because, inter alia, they can deposit as a rough film under conditions of low humidity. Although smooth films can be obtained if the humidity is controlled within narrow limits, this often is not possible in industrial applications without incurring considerable expense.
The paint application process in an automotive industrial paint shop consists of four steps: storage in a tank; circulation in pipelines; spraying via a bell and/or a spray gun nozzle; and film formation on the surface of the substrate. The shear rates active on the paint in each of the steps are quite different and require varying paint rheological properties for each step. To design proper paint viscosity in waterborne automotive coatings, a distinct rheology profile is needed to provide good sprayability, sag resistance and levelling properties simultaneously. In basecoat compositions, shear thinning flow behavior is usually preferred. In many cases, special rheology control agents are used in coating formulations to provide the desired flow behavior.
Microgels or crosslinked microparticles have been used in the paint industry to improve the rheological properties of coating compositions as well as the physical properties of the coating, such as tensile strength, solvent resistance, and gas permeability. A particular goal has been to provide good sprayability, sag resistance and leveling properties simultaneously. In basecoat paints, proper shear thinning flow behavior is required to achieve this goal.
Basecoat coating compositions containing “effect” or reflective pigments, such as metallic flake pigments e.g., aluminum flake and micaceous pigments have increased in popularity in recent years because of the “glamorous” and distinctive chromic effects they provide. In such coatings, orientation of the aluminum flakes parallel to the surface of the substrate produces a unique metallic effect often referred to as “flip-flop” or “flop”. A higher flop effect provides a more desired, brighter metallic appearance having a high level of color transition or “travel” with changes in viewing angle. The rheological properties of the coating composition, especially as affected by microgels, can greatly impact the flop property by promoting proper metallic flake orientation.
U.S. Pat. No. 6,291,564 to Faler, et al. discloses an aqueous coating composition that includes a crosslinkable film-forming resin and polymeric microparticles. However, under certain application conditions, the coating compositions can provide less than optimal appearance properties when metallic flake pigments are included in the coating composition. For example, the coatings can be prone to mottling (that is, an uneven distribution of metal flakes in the cured film) and sometimes do not have a smooth appearance. Furthermore, the resulting aqueous coating may include an unacceptable level of HAPs in the form of organic solvents.
Hong et al., “Core/Shell Acrylic Microgel as the Main Binder of Waterborne Metalic Basecoats”, Korea Polymer Journal, Vol. 7, No. 4, pp 213–222 (1999) discloses an alkali swellable core/shell acrylic microgel emulsions having a hydrophobic core and a shell that included low levels of 2-hydroxyethyl acrylate and/or methacrylic acid as well as up to 8% crosslinking monomer content. The microgels provide pseudoplastic or shear thinning behavior in aqueous metallic basecoats. Addition of an alkali is required to promote swelling of the microgel, which can be problematic in achieving reproducible rheological properties.
Polymeric microparticles may be prepared by latex emulsion polymerization, where a suitable crosslinking monomer is included in the dispersed, water insoluble monomer phase. The macroscopic interactions and kinetics of latex emulsion polymerizations are generally described by the Smith-Ewart model. In the latex emulsion polymerization technique, water-insoluble or slightly water-soluble monomers are added to an aqueous continuous phase and form dispersed monomer droplets. A very small fraction of the monomers go into solution and form monomer micelles. A free radical source is added to the emulsion and polymerization is initiated within the micelles, to which additional monomer is fed from the monomer droplets. The end result is polymer particles dispersed in an aqueous continuous phase. See Principles of Polymerization, Second Edition, Odian, Wiley-Interscience, pp. 319–331 (1983).
When water-soluble monomers are incorporated into the monomer mix in a latex emulsion polymerization process, initiation of polymerization in the aqueous continuous phase can result. When water-soluble monomers are polymerized in the aqueous continuous phase of a latex emulsion polymerization, the resulting polymer typically ranges from grit or coagulum to a thick solution or gel, rather than a dispersed polymer particle. The risk of such adverse results has limited the use of water-soluble monomers in latex emulsion polymerization processes.
U.S. Pat. No. 5,102,925 to Suzuki, et al. discloses an air-drying paint composition that includes internally cross-linked polymer microparticles, a film-forming resin and a volatile organic solvent. The use of thermosetting resins in the paint composition is not disclosed. The microparticles are produced by emulsion polymerization of ethylenically unsaturated monomers and at least one crosslinking monomer in the presence of an emulsifier.
U.S. Pat. No. 4,705,821 to Ito, et al. discloses an anticorrosive metal surface pretreating composition that includes an aqueous emulsion of hard polymer microparticles and a water soluble chromium compound. The polymer microparticles are prepared by emulsion polymerization of mono-unsaturated monomers and polyfunctional monomers.
European Patent Application No. 0 358 221 to Grutter et al. discloses electrodeposition coatings that include an aqueous dispersion of a cathodic or anodic deposition resin and polymer microparticles. The polymer microparticles include 0.1 to 5%, less than 2% by example, of monomers containing hydrophilic groups.
Generally, the known microgel thickeners used in aqueous basecoat coating compositions are deficient in that the resultant basecoatings can be susceptible to penetration by a solvent-based clear topcoat into the cured basecoat (commonly referred to as “soak in” or “strike in”) and typically are only effective with certain limited clearcoats. Further, additional rheology modifiers or thickeners are often required to ensure a desired rheological profile for the cured coating composition, which also typically include HAPs solvents. Even with the additional rheology modifiers, these coating compositions can exhibit poor flow properties resulting in spray application difficulties and/or sagging upon application; moreover, these coatings can exhibit mottling, and/or a rough appearance.
In order to overcome the surface roughness of the coating, dispersions of hydrophobic polyurethanes have been added to coating compositions containing microgel thickeners in order to provide a smoother appearance. Polyurethane dispersions have been used in aqueous coating compositions as for example in U.S. Pat. No. 5,071,904 to Martin et al., which discloses a waterborne coating composition that includes a dispersion of polymeric microparticles of a hydrophobic polyurethane and is adapted to be chemically bound into the cured coating composition. The aqueous medium of the microparticle dispersion is substantially free of water-soluble polymer.
U.S. Pat. No. 4,880,867 to Gobel et al. discloses an aqueous coating composition that includes a film-forming material based on water dilutable binders which are a mixture of a hydroxyl group-containing polymer resin; a chain extended polyurethane dispersion and pigments.
U.S. Pat. No. 5,569,715 to Grandhee and U.S. Pat. No. 6,025,031 to Lettman et al. disclose coating compositions that include an aqueous dispersion, prepared in a single- or multi-stage method, of a hydrophilicized polymer resin based on a hydrophobic polyurethane resin. The coating composition is useful for the finishing of automobile bodies and plastics parts and for automotive refinishing.
U.S. Pat. No. 6,281,272 to Baldy et al. and U.S. Pat. No. 6,291,564 to Faler et al. disclose waterborne coating compositions that include a dispersion of polymeric microparticles. The microparticles are prepared by mixing monomer(s) and a chain-extended hydrophobic polyurethane together to form a pre-emulsion and particularized into microparticles by subjecting the pre-emulsion to high-shear stress using a homogenizer. The ethylenically unsaturated monomer(s) are then polymerized to form polymeric microparticles which are stably dispersed in the aqueous medium.
The polyurethane dispersions are useful for providing smoothness to the cured coating composition. However, the brightness of the coating is typically degraded, i.e., a coating containing the hydrophilic polyurethane dispersion will not be as bright as a coating that does not contain the hydrophilic polyurethane dispersion. This is especially true when effect pigments, such as those based on metal flakes, are used in the coating composition.
It would be desirable to provide a thermosetting waterborne coating composition that contains metallic flake pigments, which is useful as an original finish, contains low or no VOC or HAP materials, and has an optimal shear thinning flow profile while providing desirable appearance properties, to yield a smooth appearance, with high flip-flop, and no mottling.